I have a S3 with HED Jet 6+ rims. They are AWESOME. On the Jet + line... The Brake surface is 25mm wide and the ride on them... IMHO.. second to none. You can run much lower air pressures with these and the rolling resistance with the wider tire (I run a 23mm Continental 4000 SII... That on a 25mm rim actually measures 27.42mm) The HED Jet 6+ is out of your budget but the HED ARDENNES LT+ are not at 900.00 US.
To be fair... I am a HED dealer. But... I owned 2 sets of Jet+ HED's before I became a dealer. I was so impressed with them as a consumer. I became a dealer for them. The picture of my P5 is the way they come now with the Black decals.
There is a lot of videos on the net talking about the benefits of lower pressures. (In fact HED has a 100 PSI Max limit labeled on the wheels.)
My S3 now has a Quarq powermeter. The P5 came with it. There is a member here that has a Stages that I met at COR. He really likes it. I would have gone that way but Stages does not make one for the FSA crank I was using. I am very fond of the pedal system I have so the pedal system Power meter's were not an option for me.
I think both of the options to upgrade are great choices. Good luck with the project!